Site-specific delivery of drugs to diseased cells can lead to increased therapeutic effects and to a significant reduction of toxicity. Drug targeting by antibody-conjugated liposomes or immunoliposomes [1] represents a technology which has been applied to the targeting of specific sites of drug action such as the brain [2], lung [3], cancer cells [4–9], HIV-infected cells [10–12] or cells of the immune system [13]. Site specific targeting is mediated by the high affinity binding of monoclonal antibodies, i.e. the targeting vector, to their specific antigens. The efficacy of the method depends thereby, first, on the target specificity of the vector and, second, cellular uptake and intracellular delivery of the liposomal load. In vivo studies have shown that conventional (immuno)liposomes are rapidly removed from the circulation by cells of the reticuloendothelial system, i.e. tissue-resident phagocytes present in a number of organs, particularly the spleen and the liver [17, 18]. This interaction is not observed with sterically stabilized liposomes, which are prepared by insertion of monosialoganglioside (GM1) [19] or poly(ethylene glycol) (PEG) [20] derivatized lipids within the lipid bilayer of conventional liposomes. Such liposomes coated with inert polymers show a substantial improvement in their blood circulation halflife (in human in the order of days as opposed to minutes for conventional liposomes [21]).
Sterically stabilized immunoliposomes are often prepared by coupling of antibodies to the distal ends of PEG chains [2, 5, 22]. Using the PEG chains as linker between the liposome and antibody leads to an enhanced antibody-antigen binding since the antibody is not shielded by the steric barrier activity of PEG. Several covalent coupling methods have been developed for attaching (derivatized) antibodies at the PEG terminus. They make use of functionalized PEG-lipids with a chemically reactive endgroup such as hydrazide, N-(3′-(pyridyldithio)proprionate [23], maleimide [2], succinyl [24], p-nitrophenylcarbonyl [25] or cyanuric chloride [26]. The drawback of these chemical coupling strategies consists in the difficulty to obtain reproducible and high coupling efficiencies [26]. Slow hydrolysis of the reactive PEG-derivative may, for example, compete with antibody binding. This may result in loss of antibody and makes it necessary to determine in separate experiments the amount of bound antibody for each batch of immunoliposomes.